The present invention is directed to a plasma processor, and more specifically, to an apparatus to reduce or eliminate plasma lighting inside a gas line in a strong RF field in the plasma processor. More particularly, the present invention uses RF shielding and gas flow restricting to reduce or eliminate plasma lighting in a gas line used to deliver gas to cool the work piece in the plasma processor.
Vacuum processors for processing a work piece (i.e., etching materials from or depositing materials onto the work piece) typically include first and second ports respectively connected to a vacuum pump and one or more sources of ionizable gas. The gas is excited to a plasma in the chamber by an electrical source including a reactance responsive to a first AC source, typically an RF or microwave source. A first matching network is usually connected between the first AC source and the reactance for exciting the plasma. If the source is an RF source, the reactance is either a coil for supplying magnetic and electric fields to the chamber interior via a dielectric window or a parallel plate capacitive arrangement for supplying an electric field to the chamber interior.
The work piece, which is typically a semiconductor wafer or a dielectric sheet or a metal plate, is clamped in place on a work piece holder, i.e., chuck, that frequently includes an electrode covered by a dielectric. DC voltage is typically applied to the electrode to provide an electrostatic clamping force to hold the work piece in situ on the holder. The work piece is usually cooled by applying a coolant agent, such as helium, to a recess in the chuck. To accelerate ions in the plasma to the work piece, a second AC source is connected to the electrode by way of a matching network. Each matching network includes a pair of variable reactances having values that are varied by motors, typically step motors.
Sensors for electric parameters associated with the plasma, as coupled to the excitation reactance and as coupled to the chuck electrode, derive signals which assist in controlling the values of the variable reactances. Pressure and flow rate transducers respectively in the chamber and in a line supplying gas to the second port derive signals which assist in controlling the vacuum pressure in the chamber and the flow rate of gas flowing into the chamber through the second port.
A controller including a microprocessor and a memory system including a hard drive, random access memory (RAM) and a read only memory (ROM), responds to the signals derived by the transducers and signals from an operator input console to produce signals for controlling the variable reactances, output power of the two AC sources, the vacuum pressure in the chamber and the flow rate of gases supplied to the chamber through the second port. The memory system stores several recipes, each in the form of signals representing various parameters controlling the deposition and etching of the work pieces for differing situations. The parameters of each recipe are, inter alia, gas species to be supplied to the chamber, flow rates of the species, vacuum pressure in the chamber and output powers of the two AC sources. Each recipe can include other parameters, such as a time for carrying out each recipe and/or the thickness of the layer being deposited. The controller responds to the parameters of the recipe to control valves for the flow of the gases into the chamber, the chamber pressure, as well as the output power of the first and second AC sources. During processing, the controller controls the reactances of the first and second matching networks so that there is an efficient transfer of power between the first and second AC sources and the loads they drive so the impedances seen looking into the output terminals of the first and second sources are substantially equal to the impedances the first and second sources respectively seen by looking from their output terminals into cables connected to the first and second matching networks.
The gas line used to deliver the cooling agent, such as helium gas, is exposed to the RF source. One problem that occurs is that the helium gas exposed in the gas line to the strong RF field can become plasma in the gas line, a phenomenon called plasma lighting. Plasma can enter the vacuum chamber causing damage to the vacuum processing system and the work piece. Also, the plasma can melt the gas line thereby damaging the vacuum system and the work piece.
To reduce plasma lighting in the helium gas line, a conventional plasma arrestor can be used. A plasma arrestor is typically a one to one and a half-inch long ceramic insert which is inserted adjacent to the RF hot junction. For 200-300 mm diameter work pieces due to the strong RF field, higher cooling requirement for 200-300 mm diameter work pieces requires high gas pressure and irregular geometry for the gas line and the conventional plasma arrestor does not work and plasma lighting occurs in the gas line. Thus, a need exists for an apparatus which can reduce or eliminate plasma lighting in a strong RF field.
The present invention is directed to a plasma processor, and more specifically, to an apparatus to reduce or eliminate plasma lighting inside a gas line in a strong RF field in the plasma processor. More particularly, the present invention uses shielding and gas flow restricting to reduce or eliminate plasma lighting in a gas line used to cool the work piece in the plasma processor.
Without shielding and a gas flow restrictor plasma light can occur in the gas line used to cool the work piece. Helium is the typical gas used to cool the work piece. As used in the present invention, shielding is a partition or box for blocking an electric field from entering the space within the shield. Shielding is accomplished in the present invention by using a metal enclosure called an RF shield to surround a portion of the gas line. RF current flows on the surface of the enclosure, but does not penetrate the interior. The thickness of the RF shield is greater than the skin depth to prevent the RF field from penetrating the RF shield. However, portions of the gas line cannot be surrounded by the RF shield because portions of the gas line are connected to elements that are RF hot.
To prevent RF lighting in an exposed portion of the gas line, a flow restrictor is installed therein to increase the breakdown voltage of the gas flowing in the gas line. The interface and/or overlap of RF hot and ground are to be determined by space limitations, the strength of the field, and other factors. A corona ring is applied at sharp points/edges of the metal enclosure to reduce the voltage gradient to reduce the possibility of plasma lighting and arcing. The corona shield also reduces the voltage gradient and shields specific areas from the electric field.
In accordance with the teachings of the present invention, a plasma processor having a gas line used for cooling a work piece is provided with an RF shield and a gas flow restrictor which prevents or eliminates plasma light in the gas line.
The vacuum processor for processing a work piece according to the present invention includes a vacuum plasma processing chamber, a coil responsive to an RF source for exciting the gas to a plasma capable of processing the plasma on a holder, a gas line connected to the chamber, an RF shield surrounding a portion of the gas line, and a flow restrictor inside an unshielded portion of the gas line.
According to an aspect of the present invention, the thickness of the RF shield is greater than the skin depth to prevent the RF field from penetrating the RF shield.
According to another aspect of the invention, the gas flow restrictor having the geometry used in the present invention has small diameter flow paths which have higher breakdown voltages than larger diameter flow paths for the gas flowing in the gas line, according to the Patchen curve.
Another aspect of the present invention is a bendable flow restrictor positioned in a bent portion of the gas line which includes stiff insulate material such as a ceramic insert and insulate wool.
Another aspect of the invention includes multiple inserts to form a flow restrictor which resolves the problem of manufacturing long inserts and also reduces the feature size.
Still other objects and advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein the preferred embodiments of the invention are shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description thereof are to be regarded as illustrative in nature, and not as restrictive.